1. Field of the Invention
This invention pertains to radiometers and more particularly to balancing radiometers.
2. Prior Art
Devices for measuring incident electromagnetic radiation, commonly referred to as radiometers, are well known and are extensively used to measure incident solar radiation. The most commonly used radiometer is the pyrannometer which measures the temperature difference between silvered and blackened surfaces that are exposed to the sun. The silvered surface reflects the incident solar radiation and thus remains at the ambient temperature. The blackened surface, however, absorbs the incident solar radiation and becomes warm. It is apparent that the temperature difference between the two surfaces is due solely to the incident solar radiation absorbed by the blackened surface. Thus, the pyrannometer measures the temperature difference and correlates this to the incident solar radiation. The accuracy of the device is dependent upon the existence of a fixed relationship between temperature difference and incident radiation. Unfortunately, however, this relationship is not reliable. Thus, on cold and/or windy days, the temperature difference between the two surfaces is reduced for any given radiation intensity. This necessitates the use of correction charts. It is possible to reduce this source of error by enclosing the two sensors in a transparent dome. Unfortunately, the dome produces selective reflection and absorption of the incident electromagnetic radiation and in addition results in artificial heating of the dome interior, which factors introduce further error. The most serious shortcoming of the pyrannometer, however, is that it is precalibrated on the assumption that there is a linear relation between temperature difference and radiative flux. There appears to be neither theoretical nor empirical justification for this premise, and accordingly all the measurements made by the device are subject to doubt.
A partial solution to these problems is found in U.S. Pat. No. 3,564,252 issued to Stoft. While the Stoft radiometer senses energy rather than temperature difference it too has a number of potential sources of error. For example, the interior reflecting converging sidewalls of the chamber will introduce an error due to selective absorption and reflection similar to the error introduced by the transparent dome in the Eppley device described above. Moreover, the Stoft device requires an isothermal chamber as well as a complex chopper assembly thus reducing its portability and increasing its cost.